I don't know about the Canadian models, but didn't Chevrolet start using painted steel caps (as well as bumpers, the dash trim, etc) in '52? Before that I would say they were a stainless shell, because my originals never had any rust on them.

-Tim
'51 Chevy 3104 w/'56 235, 848 Head, HEI Dizzy, Corvette Cam, Split Manifold and Dual Carter YF's